Both organic and inorganic adhesives are used to bond semiconductor dies to various substrates during the assembling of electronic devices. Traditionally, the attachment of the individual die is accomplished by one of three distinct types of attachment: eutectic bonding, solder pre-form, and organic adhesives applied in either film form or as dispensed pastes.
Pre-form bonding involves the use of a small piece of special composition material such as soft solder material that will adhere to both the die and the package. A solder pre-form is placed on the die attach area of a substrate and caused to melt. The die is then scrubbed across the region until the die is attached, and then the assembly is cooled. In another variation of this bonding method solder wire is used to form the bond. Unfortunately, solder materials used for this application are often alloys containing lead, which is considered hazardous to the environment.
Eutectic die attachment involves using a eutectic alloy, such as gold-silicon or gold-germanium. In this method, a layer of the alloy is plated on the backside of the wafer. The wafer is then mounted on dicing tape, diced, and singulated into individual dies. By heating a substrate above the eutectic temperature (e.g. 370° C. for gold-silicon) and placing the die on it, a bond is formed between the die and substrate. When the temperature is lowered, the bond strength is rapidly increased as the eutectic composition solidifies, allowing the assembly to immediately proceed forward to the next process step of wire bonding and molding, thus, enabling a high throughput process flow.
The eutectic solder die bonding process is a well-established bonding technique and is widely used for small signal products with small dies such as discrete and power devices including diodes, transistors, and the like. However, the eutectic process requires the use of gold, which is expensive. Additionally, the eutectic process requires the step of plating the gold eutectic material on the back side of the wafer in the wafer fabrication process, which is a costly step not required for most semiconductor devices. Furthermore, these eutectic compositions typically have very high modulus and require extremely high processing temperatures, both of which can produce significant stress in the bond line. This stress can cause die cracking or device failure, especially with newer devices such as radio frequency (RF) power amplifier devices, since they use gallium arsenide semiconductors as opposed to the less fragile conventional silicon die. The stress in the bond line can also be problematic with devices that use thin die, which are inherently more fragile than conventional thicker die. Additionally, the stress is higher when there is a greater coefficient of thermal expansion (CTE) mis-match between the die and lead frame, as is seen when copper lead frames are used instead of, for instance, Alloy 42 which is more expensive.
An alternative to solder and eutectic bonding with lower cost and less bond line stress is therefore desirable. It has been very difficult to find eutectic bonding replacement materials that are able to match the high conductivity of the inter-metallic bond formed between the die and substrate, meet all other processing requirements, and utilize existing high throughput process equipment.
One possible replacement material is dispensed conductive paste adhesive. Dispensed paste bonding involves the use of a paste adhesive, such as an epoxy, to attach the die to the package. A drop of paste is dispensed through a dispensing head or needle onto the substrate. Then a singulated die from a wafer is picked up and attached to the dispensed adhesive. The assembly is then typically baked at an elevated temperature to cure the epoxy paste adhesive.
There are four major issues for conductive dispense paste adhesive as a replacement of solder die attach or eutectic bonding. First, as die sizes become smaller there are more dies per wafer, and the time required to dispense the paste can become a limiting factor in the production rate for package assembly.
Second, after the die is attached to the dispensed adhesive, the adhesive flows and forms a fillet around the die. The fillet effectively increases the footprint area of the die, which means that the assembled package must be slightly larger than the die itself. This extra area can be quite substantial, as tolerances must account for variations in fillet size caused by factors such as paste volume and flow variation, and misplacement of paste and die. In addition, since paste adhesives are prone to resin bleed, space must be also left to ensure that surrounding bond pads are not affected by excessive bleed. The need to include this additional surface area is counter to industry requirements for reduced cost and smaller form factors for these devices.
Third, placing a die onto a dot of adhesive paste is a delicate operation. The force and time must be controlled to ensure that the adhesive has completely covered the underside of the die and formed a uniform bond line. Tilting of the die may interfere with the wire-bonding process or negatively impact device reliability by creating regions of high stress where the bond line is thinnest. Paste adhesive sometimes do not dispense uniformly, leading to uneven or inconsistently dispensed dots on the die bonding pads which can cause immediate or long-term electrical failure of the semiconductor device. This is a particularly problematic issue as die become smaller because the dispensing operation and the placement operation are both more difficult to control with smaller die.
Fourth, the conductivity of available adhesive formulations has not been adequate to replace solder and eutectic bonding materials.
The first three limitations listed above for dispensed adhesives have been overcome in recent years through the development of processes and adhesive formulations for wafer backside coating (WBC). Typically the wafer backside coating is a printable, B-stage-able adhesive formulation which is coated on the backside of the wafer by screen or stencil printing. After printing, the coated wafer is heated to evaporate solvent and/or partially advance the resin, so that the coating is hardened to a non-tacky state. The wafer is then laminated onto dicing tape, diced, and singulated into individual dies with an adhesive layer on the die backside. The die can then be attached to a substrate using heat and pressure. After die attach, the adhesive is typically cured in either a snap or oven cure process.
The WBC method largely overcomes the first three limitations mentioned for the adhesive dispense process, enabling a high throughput process with uniform adhesive coverage, minimal die tilt, and limited (if any) tolerances required for fillet and bleed of the adhesive.
The WBC process is also advantageous compared to the eutectic bonding process because it utilizes the exiting manufacturing infrastructure but does not require the costly gold plating step in wafer fabrication. Additionally, organic adhesives such as those used in the typical WBC process can be easily tailored to have lower stress than gold-silicon eutectic compounds, since they require lower processing temperatures to form a bond and have lower inherent modulus.
Adhesives for WBC must meet several performance requirements. First, they must have rheology that enables printing (the specific rheology that is required varies by application, depending on such factors as the force to be applied during printing and the thickness of the coating to be applied). Second, after print and B-stage the coating must have a smooth surface, with very low surface roughness (Rz) values being desirable, and targets of less than 10 microns typical (though the specific Rz value required will depend upon the die size, thickness of adhesive on the dicing tape, and other process-specific variables). A smooth coating surface is especially important with small die because each die has a small amount of surface area in contact with the dicing tape. If the surface of the coating is too rough that contact will be poor, and will offer insufficient strength to hold the die to the dicing tape during the sawing operation, resulting in “die fly”, an undesirable phenomenon in which the die delaminates from the tape during dicing, making it impossible to pick it up for bonding to a substrate. Furthermore, a rough coating surface can result in poor bond line thickness control and “die tilt” (when the die is not flat on the substrate after die attach and cure), both of which can lead to failure of the semiconductor device. Third, after the coating has been printed and B-staged, it must be stable at room temperature so that the coated wafer may be handled and stored for some period of time (generally a few days to a few months). Fourth, the coating must have good mechanical strength to pass the dicing and pick up process.
An additional critical performance requirement is that after the adhesive has been cured it must be capable of supporting the die through the wire bonding process, without the die moving, tilting, or delaminating from the substrate. This requirement is a particular challenge with small die (less than 2×2 mm) because of the very small surface area that is in contact with the substrate, as the die tends to move very easily. Generally the adhesive must have both high modulus and high adhesion at the wire bonding temperatures (typically 200 to 320° C.) in order to support the die. A typical target for modulus is greater than 500 MPa at the wire bonding temperature to be used. Adhesion targets vary widely and depend largely upon die size.
In the situation where WBC materials are directly replacing eutectic bonding materials, the adhesive must also meet additional requirements. First, they must have very high thermal conductivity, with requirements ranging from 8-10 W/mK in some applications and 20-30 W/mK in others. Second, they must have very high electrical conductivity, with volume resistivity targets on the order of 10−5 ohm-cm. Third, they must cure very quickly (typically in less than 20 seconds) so that the cure may take place on the die bonder during die attach, with sufficient bond strength to enable the assembly to go directly to wire bonding without a time-consuming oven cure step. This enables utilization of existing bonding equipment and the high throughput typically required to replace eutectic materials. Since existing eutectic bonders can heat up to over 400° C. the substrate can be heated to temperatures that are much higher than the cure onset temperature of most organic systems, so the onset temperature of the cure is not typically critical. However, lower cure temperatures are generally preferred to minimize stress introduced into the assembly, with a target of less than 300° C. typical. If the WBC material is not replacing a eutectic material extremely fast cure may be desirable but not critical, since often an oven cure step that may take longer can be tolerated.
Traditional dispensable conductive pastes have thermal conductivity values of up to about 5 W/mK, which is inadequate for many applications, especially those where eutectic bonding materials are being replaced. Higher conductivity is sometimes achieved using one of several alternative methods. In one method the practitioner incorporates a “low melting point” metal alloy powder into a composition containing a matrix resin powder and a conductive filler that comprises at least a copper powder and graphite powder. However, this composition is not suitable for WBC for numerous reasons. First, the powder mixture of the composition is not suitable for printing. Second, the carbon fiber is too large to give satisfactory print surface. Third, the copper in this composition is susceptible to oxidation when exposed to air. Since WBC adhesives are exposed to air during printing and storage of the wafer, copper is a non-preferred filler for this application.
In a second method high conductivity is achieved by utilizing a composition of conductive metal fillers, solid resins, and fugitive solvent that does not substantially dissolve the resins. This method is not suitable for WBC, however, because it is not printable. Because the resin is not dissolved in the fugitive solvent, these formulations have an extremely high solid content in the paste composition. Typically, pastes with very high solids content have highly thioxtropic rheology, meaning that their viscosity at low shear rates is high and their viscosity at high shear rates is low. This type of behavior gives poor leveling of the adhesive after printing, since low viscosity at low shear rates is required for the adhesive to flow slightly and fill in any small lines or imperfections present after the printing process. Poor adhesive leveling typically results in high surface roughness. Furthermore, the solid resin present in this composition may be very large (up to 110 μm), which will cause a rough surface after printing.
Prior materials have been unable to meet the complex combination of performance properties required for WBC. It would be highly desirable, therefore, to have a material that can be printed and B-staged with a resulting smooth surface, has high electrical and thermal conductivity after cure, and has fast cure. In situations where the WBC material was to be used on small die it would further be desirable to have high mechanical strength at high temperatures.